1. Field of the Invention
The present invention relates to an image-sensing apparatus for sensing a color image which is provided with a solid-state image sensor having a plurality of types of color filters so as to output chrominance signals. More particularly, the present invention relates to an image-sensing apparatus that performs white balance processing on chrominance signals.
2. Description of the Prior Art
Conventionally, a solid-state image sensor that performs linear conversion by converting the amount of incident light linearly for output has a narrow dynamic range, typically a two-digit figure. Thus, when it is used to shoot a subject with a brightness distribution spread over a wide range of brightness, it does not output brightness information outside its dynamic range. As a different type of conventional solid-state image sensor, there has also been proposed one that performs logarithmic conversion by converting the amount of incident light logarithmically for output (see Japanese Patent Application Laid-Open No. H11-313257). A solid-state image sensor of this type has a wide dynamic range, typically a five- to six-digit figure. Thus, when it is used to shoot a subject with a brightness distribution spread over a considerably wide range of brightness, it can convert all the brightness information within the brightness distribution into an electrical signal for output. However, with this solid-state image sensor, its shootable brightness range is so wide as compared with the brightness distribution of the subject that a region with no brightness data appears in a low- or high-brightness region within the shootable brightness range. To overcome these inconveniences, the applicant of the present invention has proposed a solid-state image sensor that is switchable between linear and logarithmic conversion as described above (see Japanese Patent Application Laid-Open No. 2002-77733).
In an image-sensing apparatus provided with such a solid-state image sensor, when the shooting of a color image is achieved by the provision of color filters, the spectral distribution of the light source used for shooting and the differences in transmissivity among the color filters for different colors cause different photoelectric conversion characteristics for different chrominance signals. For this reason, in an image-sensing apparatus that shoots a color image, white balance processing is performed to make the photoelectric conversion characteristics for different chrominance signals identical. For this purpose, the applicant of the present invention has proposed an image-sensing apparatus provided with a solid-state image sensor that performs logarithmic conversion wherein white balance processing is performed by a white balance processing circuit on the basis of the color temperature detected by a color temperature detection circuit (see Japanese Patent Application Laid-Open No. 2002-10275). The applicant of the present invention has also proposed an image-sensing apparatus provided with a solid-state image sensor that performs logarithmic conversion wherein white balance processing is performed by switching offset voltages when an A/D converter performs A/D conversion (see Japanese Patent Application Laid-Open No. 2002-290980).
However, in the image-sensing apparatuses proposed in Japanese Patent Applications Laid-Open Nos. 2002-10275 and 2002-290980, while white balance processing is effective for an image signal outputted from a solid-state image sensor that performs photoelectric conversion only with logarithmic conversion characteristics, it does not work with a solid-state image sensor that performs photoelectric conversion with linear conversion characteristics. Thus, when a solid-state image sensor that can automatically switch between logarithmic and linear conversion characteristics as proposed in Japanese Patent Application Laid-Open No. 2002-77733 is used, it is not possible to obtain a proper white balance by white balance processing using only multiplication and division or addition and subtraction.